The LIDAR (Light Detection and Ranging) technique is conventionally used for detecting and measuring the concentration of a given gas. Lidar may also be used for detecting liquid or solid particles in suspension in the atmosphere or further for detecting compounds dissolved in a liquid. Lidar consists of sending laser pulses into the medium of interest and of measuring their back-scattering versus time. The fact that the laser is pulsed enables detection as a function of time t and therefore of the distance z between the laser and the measuring point (z=c·t/2, c being light velocity in the medium).
Remote detection by Lidar of compounds is often carried out by the DIAL technique in which a pair of close wavelengths is used, respectively adjusted on an absorption band of the compound to be detected and immediately nearby (differential absorption). This technique is only applicable to compounds having at least one fine absorption line in a range where no other potentially present compound has an absorption band. Indeed, it is sensitive to the interferences of other compounds which absorb in the same range of wavelengths. Moreover, depending on the ranges of wavelengths, it may be difficult to produce a tunable monochromatic source adapted to the measurement.
When the spectrum of the compound to be measured does not allow application of the DIAL technique, correlation spectroscopy may be applied. Correlation spectroscopy consists of using a light source with great spectral width, modulated upon crossing a reference sample containing the compound to be measured. But this technique lacks flexibility since, for each measurement, the adapted reference has to be available. Further, the intensity intended to be absorbed by the compound to be measured is pre-attenuated in the reference sample. In order to obtain a sufficient measured signal, it is therefore necessary to have an intense light source, which may pose eye safety problems both for the operators and for the general public.